The thermal cracking of hydrocarbons including gaseous paraffins up to naphtha and gas oils to produce lighter products, in particular ethylene, has developed commercially as the pyrolysis of hydrocarbons in the presence of steam in tubular metal coils disposed within furnaces. Studies indicate that substantial yield improvements result as temperature is increased and reaction time is decreased. Reaction time is measured in milliseconds (ms).
Conventional steam cracking is a single phase process in which a hydrocarbon/steam mixture passes through tubes in a furnace. Steam acts as a diluent and the hydrocarbon cracks to produce olefins, diolefins, and other by-products. In conventional steam cracking reactors, feed conversion is about 65%. Conversion is limited by the inability to provide additional sensible heat and the heat of cracking in a sufficiently short residence time without exceeding TMT (tube metal temperature) limitations. Long residence time at high temperature is normally undesirable due to secondary reactions which degrade product quality. Another problem which arises is coking of the pyrolysis tubes.
Such steam cracking process, referred to as "conventional" hereinafter, is described or commented on in U.S. Pat. Nos, 3,365,387 and 4,061,562 and in an article entitled "Ethylene" in Chemical Week, Nov. 13, 1965, pp. 69-81, which are incorporated by reference.
In contradistinction to coil reactors in which heat transfer is across the wall of the coil and which thus are TMT-limited crackers, methods have also been developed that use hot recirculating particulate solids for directly contacting the hydrocarbon feed gas and transferring heat thereto to crack the same.
Methods in this category, designated TRC process, are described in a group of Gulf/Stone and Webster patents listed below which, however, are limited to longer residence times (50-2000 ms) and conventional temperatures, as compared with the present invention.
______________________________________ U.S. Pat. Nos.: ______________________________________ 4,057 490 4,309,272 4,061,562 4,318,800 4,080,285 4,338,187 4,097,362 4,348,364 4,097,363 4,351,275 4,264,432 4,352,728 4,268,375 4,356,151 4,300,998 4,370,303 European Application 80303459.4. ______________________________________
It should be noted that U.S. Pat. No. 4,061,562 in column 2, states that there is little or no slippage between the inert solids and the flowing gases (slip is the difference in velocity between the two). A similar connotation is found in U.S. Pat. No. 4,370,303, column 9, which cautions against gas at above 125 to 250 ft./sec. because then erosion is accelerated. Lowering gas velocity makes other steps slower also, for example, separation of solids from gas, thus adds to overall residence time. Further, one may reach a point in restricting gas velocity where good mixing of solids and gas is not achieved because high gas velocity causes turbulence and intimate mixing which are desirablfe. In a sense this invention uncouples the gas velocity from the solids velocity, that is, the former does not have to be geared to the latter in order to avoid erosive solids speed but rather the gas velocity can be relatively high and still avoid that result.
Other patents of general interest include:
______________________________________ U.S. Pat. Nos.: ______________________________________ 2,432,962 2,878,891 2,436,160 3,074,878 2,714,126 3,764,634 2,737,479 4,172,857 4,379,046 4,411,769 ______________________________________